Effects of Frequency, Percolation, and Axisymmetric Microstructure on the Electrical Response of Hot-Pressed Alumina–Silicon Carbide Whisker Composites

The electrical and dielectric properties of hot-pressed composites containing alumina and silicon carbide (SiC) whiskers were characterized over a wide frequency range (0.1 Hz–1.8 GHz). The results were correlated to the average distances between SiC inclusions which were measured by stereology as a function of orientation and composition. Percolation of the whiskers caused a drastic increase in the dc conductivity and the prominence of a dc-conductivity tail associated with a high-frequency Maxwell–Wagner interfacial polarization. In percolated samples, the tail obscured the dielectric loss peak and there was evidence for the fluctuation-induced tunneling mechanism of conduction. In nonpercolated samples, the loss peak was observed and the complex permittivity data were fit with a modified Maxwell–Wagner equation to account for a distribution of relaxation times. The frequency–dispersion magnitudes, fitting exponents, and central relaxation times were orientation-dependent. Also, a damped resonance was observed between 1.4 and 1.7 GHz. The influence of the results on the microwave-heating application is discussed.